An active antenna system (AAS) using vertically arranged antenna elements is being discussed. Implementation of three-dimensional multiple input multiple output (3D-MIMO) with antenna elements arranged in the horizontal and vertical directions is also discussed. In these antenna architectures, it may be expected that each of the antenna elements is connected to associated one of transmission/receiving circuits and that the phases and the amplitudes of signals are controlled individually. In this scenario, antenna ports may be set up corresponding to the respective antenna elements, or an antenna port may be allocated to a group or a combination of two or more antenna elements.
In general, an antenna port corresponds to a MIMO branch; however, multiple antenna ports may be divided into groups such that each group corresponds to a MIMO branch. When multiple antenna ports are provided, different antenna configurations can be produced depending on how the antenna ports are grouped. For example, a multimode antenna in which antenna elements are grouped into sections for data transmission/reception depending on the communication type such as scheme, rank, or the number of users to be multiplexed is proposed. See, for example, Patent Document 1 listed below.
Long term evolution (LTE) systems provide two types of MIMO transmission. In single user MIMO (SU-MIMO) transmission, data sequences transmitted simultaneously from different antenna branches are addressed to the same user. In multi-user MIMO (MU-MIMO) transmission, data sequences transmitted from multiple antenna branches are addressed to two or more users.
In single user MIMO, transmission data sequences are transmitted simultaneously and parallel to each other by controlling the phases and the amplitudes of signals of the respective data sequences for the associated antenna branches. Alternatively, antenna branches with less mutual correlation may be selected to transmit the data sequences simultaneously and in parallel. In multi-user MIMO, inter-user interference is reduced to achieve simultaneous and parallel data transmission by controlling the phases and the amplitudes of signals of different transmission antenna branches for the respective users, or for the respective users and transmission data streams. Alternatively, antenna branches with less mutual correlation may be used to transmit data to two or more users simultaneously and parallel to each other.
In SU-MIMO scheduling, an ordinary scheduling scheme such as proportional fairness, round robin or Max CIR may be employed by determining a rank based upon the receiving quality. On the other hand, in MU-MIMO scheduling, it is proposed to implement optimum resource allocation taking the receiving quality into account when spatially multiplexing user data items using M antenna elements over one resource block. See, for example, Patent Document 2 listed below.
With the scheduling technique described in Patent Document 2, the scheduling process is implemented every moment independently. When the positional relationship changes between travelling user equipment units, fluctuation in the user equipment becomes large and the temporal robustness for scheduling against the positional change is low. For this reason, resource allocation is updated frequently and the scheduling workload increases. Besides, orthogonality is calculated for all the combinations of users with round robin for each antenna configuration based upon the channel estimation results responsive to the number of antenna ports, and then comparison is made. The amount of calculation dramatically increases. Assuming increase of the number of antenna ports and the number of antenna configurations in coming years, a practical method for scheduling users and antenna configurations has not been established.